1) Field of the Invention
The present invention relates to a semiconductor laser apparatus, a semiconductor laser module and an optical fiber amplifier using the semiconductor laser apparatus, and a usage determining method for a semiconductor laser apparatus.
2) Description of the Related Art
Recently, as optical communications represented by the Internet develop, the arrangement of an optical fiber amplifier midway along a transmission optical fiber so as to transmit an optical signal over a long distance is widely adapted. The optical fiber amplifier amplifies signal light the intensity of which is decreased in the course of the transmission of the signal thereof in the optical fiber and thereby restores the intensity of the signal light. Specifically, an amplification system using an Erbium doped fiber (to be referred to as “EDF” hereinafter), a system using Raman amplification and the like are proposed and put to practical use. In addition, in order that signal light is amplified midway along a transmission submarine optical fiber, a so-called remote pump type optical fiber amplifier is proposed. The remote pump type optical fiber amplifier has a pump source is installed on land and transmits pump light from the pump source to the EDF via a pump light transmission optical fiber. The remote pump type optical fiber amplifier can also easily maintain by installing the pump source on land.
An increase in the amplification gain of an optical fiber amplifier leads to a decrease in the number of optical fiber amplifiers to be arranged and thus signal light can be transmitted at lower cost. Each optical fiber amplifier includes a semiconductor laser apparatus that functions as a pump source. Light amplification gain of signal light normally corresponds to the intensity of a laser beam emitted from the semiconductor laser apparatus provided in the optical fiber amplifier. To realize an optical fiber amplifier having high amplification gain, therefore, it is desirable that the semiconductor laser apparatus used as the pump source has high optical power. For this reason, the development of high power semiconductor laser apparatuses is actively underway and semiconductor laser apparatuses that have sufficiently high optical power to each serve as a pump source have been partially realized.
However, as the output of the semiconductor laser apparatus that constitutes the pump source improves, a new problem arises. As explained above, pump light emitted from the pump source is incident on the transmission amplification optical fiber. However, when light having higher intensity than a certain threshold is incident on the optical fiber, stimulated Brillouin scattering occurs. The stimulated Brillouin scattering is a nonlinear optical phenomenon that scattering (reflection) occurs by the interaction of the incident light with an acoustic wave (phonon). It is observed as a phenomenon that the light with frequency about 11 GHz (giga-hertz) lower than the incident light loses energy equivalent to phonon through the scattering and is reflected in opposite direction to the incident light.
According to the optical fiber amplifier using Raman amplification, when the stimulated Brillouin scattering of the pump light occurs, some of the incident pump light is reflected backward and does not contribute to the generation of Raman gain. In addition, there is a probability that this scattered light generates unexpected noise. The decrease of the intensity of the pump light is not so serious when the distance that a pump light travels is short. However, since the optical fiber amplifier using the remote pump requires. With an optical fiber amplifier using the remote pump, the decrease of the intensity of pump light is greater than the light loss of an ordinary optical fiber. As a result, the amplification gain of the amplification optical fiber disadvantageously lowers.